1. Field of the Invention
This invention relates generally to compact, high-efficiency optical devices that concentrate, collimate, redirect or otherwise manipulate a beam or source of electromagnetic radiant energy, such as concentrators, collimators, reflectors, and couplers.
2. Description of Related Art
The field of non-imaging optics relates to optical devices that collect and concentrate light energy from a distant source onto a receiver, or to optical devices that redirect or collimate light from a closely positioned source. In one example, a photovoltaic solar energy collector utilizes non-imaging optics to increase the power density upon a receiver. Because the purpose of a photovoltaic solar collector is to convert light energy into electrical energy, it is not necessary to precisely image the sun onto the receiver; rather it is only important that the solar energy at the entry aperture be concentrated somewhere upon the receiver. In other words, the imaging characteristics of a solar energy collector are unimportant. In addition to non-imaging collectors, the field of non-imaging optics relates to optical devices that redirect, shape, and/or collimate light from a closely positioned source without regard to accurately imaging the source at a distant location. An example of one important light source is an LED (light emitting diode), which emits light in a widely-dispersed pattern with large intensity variations. A non-imaging reflector in one such application attempts to transform the non-uniform LED source into a substantially uniform output light beam. Although non-imaging optics are often unsuitable for imaging uses, some non-imaging optical designs also exhibit good imaging characteristics.
A generic problem in the field of non-imaging geometric optics is how to design very high efficiency (high efficacy) compact concentrators, collimators, redirectors and/or couplers that are cost effective and mass-producible. A number of solutions to this problem have been disclosed but they all have inherent weaknesses.
One type of non-imaging optics uses faceted total internal reflection (TIR)/refractive elements. This type of non-imaging optic is disclosed in U.S. Pat. No. 5,676,453, by Parkyn et al. entitled Collimating TIR lens devices employing fluorescent light sources, U.S. Pat. No. 5,404,869 by Parkyn et al entitled: Faceted internally reflecting lens with individually curved faces on facets, and U.S. Pat. No. 4,337,759 by Popovich et al. entitled Radiant Energy Concentration by Optical Total Internal Reflection. However, the efficiencies of the devices manufactured to date based on this approach have not been as high as predicted by the theoretical models in these patents and related publications. One reason for this low efficiency relates to difficulties associated with accurately manufacturing the required optical shapes in plastics and other optical materials. Secondly, this approach from a theoretical standpoint is not an optimum solution. Maximum efficiencies for conventional optical designs based on this approach do not typically exceed 60% and are quite often found to be 50% or less. Further, as a consequence of the complexity of the geometry of these designs, their inherent manufacturing costs can be high.
A second approach for developing non-imaging optics is the edge-ray approach, which is disclosed in Welford and Winston, High Collection Nonimaging Optics, Academic Press, New York, 1989. The edge-ray approach is particularly useful for two-dimensional designs that can be rotated to provide a rotationally symmetrical device, or extended linearly to provide a linearly symmetrical device. Designs based on this second approach are disclosed in U.S. Pat. Nos. 5,289,356; 5,586,013 and 5,816,693, for example. One characteristic device of this method is the CPC (Compound Parabolic Concentrator), which is described beginning at page 55 of the Welford and Winston reference. FIG. 1 is a cross-sectional view of a conventional CPC device, including two opposing mirror-image parabolic reflector sections 1 and 2 arranged symmetrically around a central axis 3 (the z-axis). The surfaces are formed by choosing a partial segment of a first parabola with a first focus, displacing a mirror image of this curve a distance away from the first focus, and rotating the two curves with respect to a common axis. The resulting two planar curves are either swept around a central axis or extruded along a straight line.
The CPC collect all the radiation impinging upon its entry aperture within the angle xc2x1xcex8s, and concentrates it upon a receiver 4, which is situated opposite an entry aperture with its center intersecting the z-axis. The sections of the parabolic reflectors are arranged such that their respective axes of symmetry are tilted with respect to the z-axis, which widens the entry aperture. Also, each parabola""s focus is at the opposite edge of the receiver, as illustrated at 5 and 6. Particularly, the focus of the first parabolic reflector section 1 is at the opposite edge 5, and the focus of the second parabolic reflector section 2 is at the opposite edge 6. For illustrative purposes, in one embodiment the reflectors comprise a mirrored surface, and the center of the CPC is air. In other embodiments, the CPC comprises a dielectric material with parabolic surfaces that provide an index of refraction greater than one, and the reflections from the parabolic surfaces are facilitated by total internal reflection.
In addition to the typical use of a CPC as a collector/concentrator, it can also be used as a emitter/collimator by positioning a source at the location of the receiver. The performance of a CPC can be very high for some embodiments; for example, a CPC can achieve a transmission efficiency of over 90% with acceptance angles 5xc2x0 or less. Disadvantages of a CPC are its thickness for small angles, and its difficulty of manufacture because the CPC reflectors end at the receiver (emitter) edges. This problem is particularly significant when the receiver (emitter) has a small size. The complexity of manufacturing for these types of devices can be significantly reduced by replacing the side reflectors with a solid dielectric-filled CPC that utilizes total internal reflection on its outside surface instead of reflection on its inner surface. An example of this approach is disclosed in X. Ning, R. Winston, J. O""Gallagher. Dielectric Totally Internally Reflecting Concentrators, Applied Optics, Vol. 26, (1987) pp. 300. Designs based on this approach are also disclosed in U.S. Pat. Nos. 5,055,892, 5,001,609, and 5,757,557. However, manufacturing such devices is problematic because adhesion of the receiver (emitter) to the solid device must guarantee that total internal reflection is achieved also at the reflector points close to the receiver (emitter) edges.
The CPC device is first designed as a two-dimensional (2D) cross-section, and then made into a three-dimensional (3D) design. In one embodiment, a rotationally symmetrical 3D design is obtained by rotating the cross-section shown in FIG. 1 about an axis through the centerline of symmetry. Alternatively, a linearly symmetrical 3D design is obtained by extruding the cross-section shown in FIG. 1 along a line perpendicular to the 2D cross section.
In a CPC design disclosed by A. Santamaria and F. J. Lopez-Hernandez, Wireless LAN Systems, Artech House, 1993, pp. 74, for an acceptance angle of xc2x11xc2x0, the thickness-to-aperture size ratio is greater than 40. However, for certain acceptance angles, a solid dielectric CPC (with curved top) can reduce the thickness-to-aperture ratio and improve on the thickness-to-aperture. For example, for acceptance angles of 10xc2x0 or more, this ratio can be reduced to between 1.0 and 2.0, as disclosed by Ning el al. Dielectric Totally Internally Reflecting Concentrators, Applied Optics, Vol. 26, 1987, p. 300. When the acceptance angle is smaller, the thickness of a concentrator/collimator can also be reduced by combining an image-forming device with either a CPC or one of the associated family of devices such as a CEC (compound elliptical concentrator), such as disclosed by the Welford and Winston reference. However, the optical performance of such a hybrid device is not close to the thermodynamic limit, and furthermore, the high manufacturing cost and complexity of such devices are disadvantages.
Another approach, which is useful for three-dimensional designs, is the flow line approach described by the Welford and Winston reference (Ch. 7). One very high concentration design using the flow line approach has two stages: a non-imaging Flow Line Concentrator (FLC) combined with an optical imaging device such as a parabolic reflector. FIG. 2 shows a 2D cross-sectional example of a two-stage approach, including a first optical stage that is a parabolic mirror 10 that focuses the incoming ray bundle towards a virtual receiver, illustrated generally at 12 as the area between two edge points. The rays directed toward the virtual receiver are then further concentrated by a FLC 14, which comprises the second stage that includes a smaller size receiver 16. The trajectories of two edge rays are also shown. Unlike the rotational CPC, the FLC has the property of achieving the theoretical limit of concentration in 3D-geometry, as disclosed by Welford and Winston, pages 197-199. Although this hybrid design is at useful for some applications such as optical communication links, the displacement of the imaging component from the FLC does create difficulties for manufacturing. Another disadvantage is that the unit is not compact in depth.
In the early and middle 1990s, development began on the Simultaneous Multiple Surfaces (SMS) method for the design of non-imaging concentrators and collimators. Examples of this early SMS method are disclosed in: J. C. Mixc3x1ano and J. C. Gonzxc3xa1lez, xe2x80x9cDesign of Nonimaging Lenses and Lens-Mirror Combinationsxe2x80x9d SPIE""s International Symposium on Optical Applied Science and Engineering, San Diego, Calif. Proc. SPIE 1528 (1991) pp. 104-117; J. C. Mixc3x1ano and Juan C. Gonzxc3xa1lez, xe2x80x9cNew Method of Design of Nonimaging Concentratorsxe2x80x9d, Applied Optics, 31 (1992) pp. 3051-3060; Mixc3x1ano et al., xe2x80x9cRX: a Nonimaging Concentratorxe2x80x9d, Applied Optics, 34, 13 (1995) pp. 2226-2235; and J. C. Mixc3x1ano et al, xe2x80x9cRXI: A high-gain, compact, nonimaging concentratorxe2x80x9d, Applied Optics, 34, 34 (1995) pp. 7850-7856. Advantageously, unlike the CPC or flow line devices described earlier, whose reflectors end at the receiver (emitter) edges, in SMS devices there is no optical surface in contact with the source/receiver.
Unfortunately, the early SMS method disclosed in these papers has a number of significant limitations that greatly restrict its usefulness. The early SMS method generated a design that was described only as a series of points, and not as a polynomial or any analytical expression. This discontinuous point-by-point description made it very difficult to manufacture the device, and also made it difficult to adapt a design or add features to the device. Furthermore, the second derivative was discontinuous in the early SMS method.
The early SMS method imposed very significant restrictions on the input and output ray bundles that could be accommodated, thereby limiting the usefulness of the early SMS method to mostly theoretical sources and receivers, effectively eliminating most real-world, practical applications. Specifically, the early SMS method accommodated only two types of input bundles: 1) parallel flow lines from an infinite source (for a concentrator) or 2) a ray bundle from a flat finite, isotropic or Lambertian source (for a collimator). These are very severe restrictions, effectively eliminating LEDs, light bulbs, and most other real-world light sources from design consideration. The early SMS method also could not handle embodiments in which the emitter is surrounded on one side by a reflector such as a CPC reflector cup. Furthermore, the early SMS method would not allow for the receiver (or emitter) at the focus to be embedded in a material with a different index of refraction than the material (e.g. dielectric) of the main device.
For the relatively few applications in which the early SMS method could be used, the optical concentrators and collimators or other devices designed using the method typically exhibited one or more of the following four desirable characteristics:
1) High efficiency: For small acceptance (concentrators) and output (collimators) angles ( less than 10xc2x0) these devices achieved more than 96% of the maximum theoretical attainable efficiency (based on etendue considerations).
2) Compactness: the aspect ratio (device thickness/aperture diameter) can be as small as 0.27, for small angular values of acceptance (or output).
3) The receiver or source does not have to be in contact with the optical device. Unlike the CPC or flow line devices described earlier, whose reflectors end at the receiver (emitter) edges, there is no optical surface in contact with the receiver (emitter). The TIR faceted devices share this trait with the SMS-based devices.
4) Simplicity: The devices are simple, and many embodiments can be manufactured in a single solid piece.
In light of these desirable characteristics, it is clear that it would be useful if an SMS method could be developed to design optics for a wider variety of real-world sources and receivers, such as LEDs. LEDs are low cost, high efficiency light sources, and it would be an advantage if they could be utilized for a wide variety of illumination requirements. However an LED""s emission pattern is far from the cosine law produced by a flat Lambertian emitter. The non-isotropic, non-Lambertian output from the LED is caused by several factors. First, the LED die geometry is very different from a flat disk and also the LED die itself (even with a secondary optic device) exhibits a non-isotropic, non-Lambertian output. Furthermore, in order to direct the output light in approximately one general direction, LED packages typically include a reflector cup that modifies the output profile from the bare LED die, adding more non-uniformities. Also, LED dies and reflector cups are typically bonded in place using epoxy, silicone, and other adhesives, which all contribute to produce a highly non-isotropic, non-Lambertian output.
In order to overcome the limitations of the prior art, the present invention provides an optical device that is highly efficient and a method of manufacturing the optical device in a variety of forms, such as optical devices that transform a first radiation distribution that is non-uniform into a second radiation distribution that is substantially uniform.
A method of manufacturing an optical device that has two opposing active optical surfaces that convert a first distribution of an input radiation to a second distribution of output radiation includes providing a two-dimensional mathematical model that describes the first distribution of radiation as an input bundle of edge rays and the second distribution of radiation as an output bundle of edge rays. The input and output edge ray bundles are each represented in a phase-space representation in terms of the position of each ray in space and its associated optical cosine of propagation, where the locus of the edge rays in the phase-space for the input bundle defines a closed boundary of a first planar shape, and the locus of the edge rays in the phase-space for the output bundle defines a closed boundary of a second planar shape, wherein these two planar shapes have a substantially equal area. The two-dimensional shape of the outer caustic is approximated for the input and output radiation distribution ray bundles, where the outer caustic is defined such that it does not touch any of the active optical surfaces. A two-dimensional representation of the active optical surfaces is defined responsive to the boundary conditions of the phase-space representations and the outer caustics, including defining a focal area spaced apart from, and noncontiguous with, the optical surfaces, the active optical surfaces each having a continuous second derivative. The optical surfaces are formed so that the theoretical transmission efficiency of the first input radiation distribution to the second input radiation distribution, neglecting attenuation losses in the processing path, is greater than about 80% of the maximum transmission efficiency. To form a three-dimensional optical device, the two-dimensional representation of the optical surfaces is symmetrically extended rotationally or linearly.
An optical device that converts a first distribution of an input radiation to a second distribution of output radiation comprises two opposing active non-spherical optical surfaces defined by a two-dimensional representation that is symmetrically extended to provide a three-dimensional device. A focal area is defined by the two opposing active optical surfaces. The active optical surfaces each have a continuous second derivative, and the optical surfaces are defined by a polynomial with an order of at least about twenty. The optical surfaces provide a theoretical transmission efficiency of the first input radiation distribution to the second input radiation distribution, neglecting attenuation losses in the processing path, of greater than about 80% of the maximum transmission efficiency.
In some embodiment the optical device is rotationally symmetric, and in other embodiments the optical device is linearly symmetric. The optical device may comprise a transparent dielectric core, and the optical surfaces may be formed on the optical core. If a receiver is situated approximately at the focal area of the device, a concentrator is provided. If an extended light source is situated approximately at the focal area, then a collimator is provided.
The optical device may be an RR device, or a folded edge ray device such as an RX device, an RXI device, an XX device, or an XR device.
The surfaces can have a variety of configurations. For example, one of the optical surfaces may be substantially flat. In a faceted embodiment at least one of the optical surfaces comprises facets including an active facet and an inactive facet.
Faceted devices can be made with a small aspect ratio: the faceted optical surfaces define an aspect ratio within a range of about 0.65 to about 0.1. Faceted surfaces provide other advantages, such as the heat transfer advantage of an embodiment wherein one of the active surfaces comprises a cuspoid shape that approaches the focal area. In other embodiment at least one of the optical surfaces comprises a diffuser formed thereon. The diffuser may transforms incident radiation into a predetermined shape, such as rectangular, elliptical, or a crossed shape.
In one embodiment, the optical device is rotationally symmetric and defines a central axis, and the device comprises a dielectric core that has a cylindrical hole formed on the central axis with a spherical void at its top. For a collimator, an extended light source such as an LED is inserted into the hole and situated approximately at the focal area. The LED can be attached to the dielectric core by an attaching material that has a substantially different index of refraction than the dielectric. For a concentrator, a receiver can be inserted into the hole and situated approximately at the focal area. In another embodiment in which the optical device is linearly symmetric, a linear light source such as an array of LEDs can be situated along the linear focal area to provide a linear emitter, or a linear receiver can be situated along the linear focal area to provide a linear concentrator.
Substantial advantages are provided because the active surfaces of the optical devices described herein have a continuous second derivative, and can be represented by a polynomial power series. This representation is very useful for interfacing, modifying, and adapting the design with CAD packages and CNC, and also for manufacturing it with diamond turning equipment. For example, the overall or macro shape of the transducer can be adapted to devices using micro-structured curved or flat facets, unlike the early SMS method. Faceted designs can provide a lower aspect ratios; i.e. more compact designs than has been achieved previously using the old SMS method or other non-imaging optic design approaches. As an added benefit, an infinite variety of transducers can be designed. Furthermore, the ability to represent the surfaces by a polynomial allows a solution to be chosen from a family of solutions where there is a geometric or other constraint, which was not possible with devices designed using the early SMS method. The method described herein ensures that the second derivative of the active optical surfaces is continuous, unlike the early SMS method in which it was discontinuous.